Electric conductivity and oxygen permeability of modified cerium oxides

Electrical conductivities of samarium (Sm), terbium (Tb), praseodymium (Pr) and zirconium (Zr) doped ceria membranes were measured in T = 600–900°C and pO₂ = 10^–22–0.21 atm. Doping Sm and Pr in CeO₂ respectively enhances the ionic conductivity and electron-hole conductivity of ceria. Sm-Pr doped ceria exhibits both n-type (at lower pO₂) and p-type (at high pO₂) electronic and ionic conductivity in the temperature range studied. Adding Tb in the Sm doped ceria causes a reduction in ionic conductivity. Zr-Pr doped ceria is an n-type electronic conductor at low and p-type electronic conductor at high pO₂. The ionic conductivity of Zr-Pr doped ceria is lower than Sm doped ceria but higher than pure ceria. Oxygen permeation flux through the Zr-Pr doped ceria membrane, dominated by the slow oxygen ionic conduction, is similar to the ytttria doped bismuth oxide membrane, and smaller, but close to that of perovskite-type lanthanum cobaltite membrane.     

钴掺杂氧化铈纳米粒子电催化性能研究

氧化铈的电子导电性较低、氧空位数量少, 难以单独用作为电催化剂。但是掺杂过渡金属或非金属元素可以提高氧化铈的CO催化能力, 同时在氧化物中掺杂钴可有效提高材料的电催化能力, 因此本工作开展了对钴掺杂的氧化铈电催化性能的研究。采用均相沉淀法制备了钴掺杂的氧化铈纳米粒子, 电化学测试发现当钴掺杂比例为20mol%时, 氧化铈纳米粒子对氧气还原反应(ORR)和氧气析出反应(OER)的综合催化能力最强。经过10 h的长时间催化作用, ORR、OER过程中的电流密度分别下降了20%、5%左右, 远优于贵金属和未掺杂氧化铈纳米粒子催化剂, 显示出良好的催化稳定性。拉曼光谱、阻抗图及XPS谱图等的测试分析表明钴掺杂后材料的电荷转移阻抗降低(电子导电性的提高)、氧活性物种和氧空位增加是氧化铈催化性能提高的主要原因。本工作通过钴掺杂大幅度提高了氧化铈的电催化性能, 同时为其它离子导体作为双功能电催化剂的使用提供了借鉴。     

Preparation of YSZ/YDC and YSZ/GDC composite electrolytes by the tape casting and sol-gel dip-drawing coating method for low-temperature SOFC

The composite electrolytes for low-temperature solid oxide fuel cells were fabricated via coating the YSZ sol on tape-casted substrates of yttria doped ceria (YDC) and gadolinia doped ceria (GDC). The doped ceria substrates with 98% of relative density were prepared by the tape-casting method followed by the sintering at 1,500°C for 2 hours. The YSZ polymeric sol for dip-drawing coating was synthesized by the partial hydrolysis of Zr-n-butoxide. The optimum dip-drawing coating rate for obtaining pinhole and crack free YSZ film was 2 cm/min using YSZ polymeric sol with 1.13 mol/ of concentration. After 10 times coating on ceria substrates with YSZ followed by the heat-treatment at 1,400°C for 2 hours, the fully densified YSZ film with 2.0 m of thickness without pores, cracks, or chemical reactions could be obtained. The results of single cell tests shows that YSZ layer coated doped ceria composite electrolyte prepared by the sol-gel dip-drawing method has an superior single cell performance to YSZ electrolyte without dissociation of ceria substrate.     

Synthesis of nanocrystalline yttria doped ceria powder by urea-formaldehyde polymer gel auto-combustion process

Nanocrystalline yttria doped ceria powder has been prepared by auto-combustion of a transparent gel formed by heating an aqueous acidic solution containing methylol urea, urea, cerium(III) nitrate and yttrium(III) nitrate. The TGA and DSC studies showed the combustion reaction of the gel initiated at 225 °C and completed within a short period of time. XRD spectrum of the combustion product reveals the formation of phase pure cubic yttria doped ceria during the combustion process. Loose agglomerate of yttria doped ceria particle obtained by the combustion reaction could be easily deagglomerated by planetary ball milling and the powder obtained contains particles in the size range of 0.05-3.3 μm with D₅₀ value of 0.13 μm. The powder particles are aggregate of nanocrystallites with a wide size range of 14-105 nm. Pellets prepared by pressing the yttria doped ceria powder sintered to 95.2% TD at 1400 °C.     

Nanodomain formation and distribution in Gd-doped ceria

A comprehensive study, with a combination of diverse analytical techniques, was performed to investigate nanodomain formation and distribution in gadolinium-doped ceria. It is illustrated that the nanodomain formation, originating from the aggregation and segregation of dopant cations together with associated charge-compensating oxygen vacancies, is ubiquitous throughout doped ceria. The formation of nanodomains is not limited to bulk areas as previously reported but exists at grain boundaries as well. With enhanced ordering level, such nanodomains formed at grain boundaries will decrease the ionic conductivity as a result of hindered the mobility of oxygen vacancies in doped ceria. Particularly, the nanodomains formed at grain boundaries, with strong defect interactions due to enrichment of dopants and ordered oxygen vacancies, are suggested to be another possible reason for the grain-boundary resistance, other than the widely accepted space-charge layers.     

Gadolinia doped ceria/yttria stabilised zirconia electrolytes for solid oxide fuel cell applications

Solid oxide fuel cells are currently constructed using a yttria stabilised zirconia electrolyte membrane. However, zirconia has a number of disadvantages associated with its use, such as the high operational temperatures required for it to exhibit acceptable levels of ionic conductivity. Alternative ceramics such as doped cerium oxide show promise as electrolytes capable of operating at reduced temperatures, but introduce additional problems such as electronic conduction and inferior mechanical properties. This paper describes the manufacture and characterisation of a number of prototype electrolytes consisting of a mixture of yttria stabilised zirconia and gadolinium doped ceria. Traditional ceramic processing techniques were used to produce the samples, which were then examined using dilatometry, impedance spectroscopy and X-ray diffraction. Results show a lowering of the ionic conductivity of zirconia with the addition of doped ceria. X-ray diffraction patterns obtained from the samples suggested that this effect could be attributed to the formation of a solid solution of ceria in zirconia.     

Synthesis and Characterization of SmxGdyCe1-x-yO2-δ Nanopowders

SmxGdyCe1-x-yO2-δ (x+y=0.2 and x=0, 0.04, 0.08, 0.12, 0.16, 0.2) nanopowders were prepared by a coprecipitation method. The zeta potential and sedimentation volume of Ce(OH)4 aqueous dispersions at different pH values were measured. The isoelectric point (IEP) of Ce(OH)4 suspensions is 7.0. The maximum potential value of -18.5 mV and maximum sedimentation volume of 19 ml are reached at pH=10. The evolution behaviors of the xSm(OH)3•yGd(OH)3•(1-x-y)Ce(OH)4 dried powders in the heating process was characterized by DTA/TG and XRD. The powders decompose to ceria based solid solution at a temperature below 300℃ and forms cubic fluorite structure ceria at about 650℃. The properties of SmxGdyCe1-x-yO2-δ solid solutions were characterized by XRD, TEM and BET. The lattice parameter of doped CeO2 increases linearly with increasing Sm3+ substitution (or decreasing Gd3+ substitution). The particle size of the doped ceria powders is from 5 nm to 10 nm     

Absorption and emission properties of Zr-doped nanoceria synthesised by co-precipitation-hydrothermal treatment route

A series of nanoceria samples doped with zirconium (nil to 2 mol%) and a typical sample doped with 1 mol% each of Zr and Eu were synthesised following co-precipitation-hydrothermal treatment route. The main peaks in X-ray diffraction (XRD) patterns of all the samples corresponded to (111), (200), (220) and (311) planes of cubic ceria. The Fourier transform infrared spectra showed bands which were indicative of formation of nanoceria with intercalated/adsorbed water. In Raman spectra, a sharp and intense Raman shift at around 460–462 cm^?1 was observed for all the samples and the peak intensity decreased with increase in mol% doping of Zr. In case of Zr–Eu-doped sample the peak intensity was similar to the undoped ceria. The decrease in intensity could be due to accumulation/precipitation of more numbers of Zr ions on the surface rather than incorporation into ceria lattice. The indirect band gaps estimated from ultraviolet–visible absorption spectra were smaller when compared to the direct ones. The smaller indirect band gaps indicate that Zr-doped nanoparticles with a higher fraction of atoms residing at the surface were available to assist the indirect electron transition from the valence band to the conduction band. Emission spectra obtained by exciting at 300 and 500 nm for all samples are discussed in details. Transmission electron microscope image of a typical sample showed mono-dispersed uniform sized single crystalline doped ceria.     

RETRACTED ARTICLE: Determination of dopant of ceria system by density functional theory

Oxides with the cubic fluorite structure, e.g., ceria (CeO₂), are known to be good solid electrolytes when they are doped with cations of lower valence than the host cations. The high ionic conductivity of doped ceria makes it an attractive electrolyte for solid oxide fuel cells, whose prospects as an environmentally friendly power source are very promising. In these electrolytes, the current is carried by oxygen ions that are transported by oxygen vacancies, present to compensate for the lower charge of the dopant cations. Ionic conductivity in ceria is closely related to oxygen-vacancy formation and migration properties. A clear physical picture of the connection between the choice of a dopant and the improvement of ionic conductivity in ceria is still lacking. Here we present quantum-mechanical first-principles study of the influence of different trivalent impurities on these properties. Our results reveal a remarkable correspondence between vacancy properties at the atomic level and the macroscopic ionic conductivity. The key parameters comprise migration barriers for bulk diffusion and vacancy–dopant interactions, represented by association (binding) energies of vacancy–dopant clusters. The interactions can be divided into repulsive elastic and attractive electronic parts. In the optimal electrolyte, these parts should balance. This finding offers a simple and clear way to narrow the search for superior dopants and combinations of dopants. The ideal dopant should have an effective atomic number between 61 (Pm) and 62 (Sm), and we elaborate that combinations of Nd/Sm and Pr/Gd show enhanced ionic conductivity, as compared with that for each element separately. An erratum to this article can be found at     

Sm_2O_3掺杂CeO_2纳米材料的电子顺磁共振研究

利用溶胶-凝胶法制备Sm2O3掺杂CeO2的电解质材料。通过粉末X射线衍射谱对所制备的电解质材料进行平均晶粒尺寸和晶格常数分析,并利用电子顺磁共振仪进行测试,分析样品的电子顺磁共振特性。结果表明:所制备的粉末样品晶粒尺寸较小、具有立方萤石型结构,而且Sm2O3掺杂CeO2的固体电解质材料有缺陷结构,随焙烧温度越高,缺陷结构越多;其次不同比例的Sm3+掺入CeO2后,材料中缺陷结构也有很明显的变化。     

Effect of submicron grains on ionic conductivity of nanocrystalline doped ceria

Doped ceria has been considered for high oxygen ion conductivity for solid oxide fuel cells. In the present study, 20 mole% samarium doped nano ceria powder was prepared by wet chemical synthesis and sintered at different temperatures to retain submicron grains (> 92-96% density). ionic conductivity of the sintered pellets was measured using impedance spectroscopy as a function of temperature (200-800 degrees C). The total maximum conductivity was 1.0 x 10(-2)S.cm(-1) (at 600 degrees C) for samples sintered at 1200 degrees C. The activation energy at higher test temperature decreases with the decrease in the sintering temperature (by 25%). The grain boundary, grain interior conductivity and activation energy of the electrolyte were correlated to the resulting microstructure. It has been demonstrated that use of doped nano ceria powder as precursor not only reduced the sintering temperature but also provided segregation free grain boundary for engineering higher conductivity dense electrolytes.     

Synthesis of nanocrystalline Ni/Ce-YSZ powder via a polymerization route

Pechini process was used for preparation of three kinds of nanocrystalline powders of yttria-stabilized zirconia (YSZ): doped with 1.5 mol% nickel oxide, doped with 15 mol% ceria, and doped with 1.5 mol% nickel oxide plus 15 mol% ceria. Zirconium chloride, yttrium nitrate, cerium nitrate, nickel nitrate, citric acid and ethylene glycol were polymerized at 80 °C to produce a gel. XRD, SEM and TEM analyses were used to investigate the crystalline phases and microstructures of obtained compounds. The results of XRD revealed the formation of nanocrystalline powder at 900 °C. Morphology of the powder calcined at 900 °C, examined with a scanning electron microscope, showed that the presence of nickel and cerium inhibited the grain growth in the system. The average crystallite size of the material doped with nickel oxide (9.33 nm) was bigger than the one doped with cerium oxide (9.29 nm), while the YSZ doping with the two oxides simultaneously promoted the grain growth with crystallite size of 11.37 nm. Yttria-stabilized zirconia powder with a mean crystallite size of 9.997 nm was prepared successfully by this method.     

Co-doping Effect on Microstructures and Ionic Conductivity of Aliovalent Cations Modified Ceria

Microstructural features and ionic conductivity of divalent （ Mg2 ＋ ） and trivalent （ Gd^3＋ ） cations co-doped ceria electrolyte system Ce0.8sGd0.2MgxO1. 9-s were investigated by scanning electron microscopy （SEM） and AC impedance analysis. The experimental results exhibit that addition of MgO to GDC reduces the average binding energy of GDC by decreasing the energy barrier of oxygen ion migration in ceria matrix and the ionic conductivity of 2 mol% magnesium doped GDC （0.018 S/cm） is higher than that of GDC matrix at 650℃ （0.0105 S/cm）. Co-doping Mg^2＋ and Gd^3＋ is found to increase the ionic conductivity of ceria and hence decreases the operation temperature as well as the cost of solid oxide fuel cell （SOFC）.     

Characterization of yttria-doped ceria prepared by directional crystallization

The oxygen nonstoichiometry and the related lattice defects in ceria crystals doped with the high concentration (13–26 mol%) of yttria and grown by the directional crystallization (skull technique) have been investigated. The results obtained using thermoanalysis, electrical conductivity and microhardness are described and discussed.     

Computer Modeling of Ionic Conductivity in Low Temperature Doped Ceria Solid Electrolytes

Solid oxides, such as ceria (CeO₂) doped with cations of lower valance, are potential electrolytes for future solid oxide fuel cells. This is due to the theoretically high ionic conductivity at low operation temperature. This paper investigates the feasibility of two potential electrolytes which are samarium-doped ceria (SDC) and gadolinium-doped ceria (GDC) to replace the traditional yttria-stablized zirconia (YSZ). Molecular simulation techniques were employed to study the influence of different dopant concentrations at different operation temperatures on the ionic conductivity from the atomistic perspective. Simulation results show that the optimized ionic conductivity occurs at 11.11mol% concentration using both dopants of Gd₂O₃ and Sm₂O₃. The temperature effect was also examined under a fixed concentration simulation to check how low temperature they still function. The predicted ionic conductivities have been verified with published experimental results and show reasonable agreements. This simulation technique reveals a clear picture with qualitative and quantitative connection between the choice of the dopant and the improvement of the ionic conductivity of fuel cell electrolytes.     

Synthesis of La3+ doped nanocrystalline ceria powder by urea–formaldehyde gel combustion route

Nanocrystalline ceria powders doped with various concentrations of lanthanum oxide have been prepared following gel combustion route using for the first time urea–formaldehyde as fuel. The synthesized products were characterized by XRD, FESEM, TEM, PL and UV–vis spectroscopy. Peak positions of XRD were refined and the lattice parameters were obtained by applying Cohen's method. Unit cell parameter increases with concentration of La³⁺ ion and the variation is consistently linear. XRD calculations showed the dependence of crystallite size on dopant concentrations at lower level. TEM observation revealed unagglomerated particles to be single crystals in the average range of 20–30 nm. Band gap of the La³⁺ doped ceria materials does not change with doping. Spectroscopic experiments proved the existence of Ce³⁺ in the formed powder.     

铋掺杂提高氧化铈中氧空位浓度增强CO2光催化还原性能

氧空位在CO₂光催化还原过程中往往发挥重要作用。本工作中, 用水热法合成了不同Bi掺杂量的二氧化铈光催化剂Ce_1-xBi_xO_2-δ, 其中Ce_0.6Bi_0.4O_2-δ在Xe灯照射下表现出最高的光催化活性, 其CO产率为纯二氧化铈纳米棒的4.6倍。X射线衍射(XRD)分析表明固溶体保留了二氧化铈的萤石结构;紫外-可见漫反射(UV-Vis)光谱表明固溶体可见光吸收增强;X射线光电子能谱 (XPS)和拉曼光谱(Raman)分析表明, 掺杂后氧空位浓度明显提高。结合原位傅里叶变换红外光谱(in-situ FT-IR), 发现引入Bi提高了固溶体中氧空位的浓度, 并改变了CO₂在催化剂表面上的吸附/活化行为, 光照下碳酸氢根、碳酸根、甲酸等中间产物明显增多, 从而增强了CO₂光催化还原性能。     

Characterisation of Ce0.8Gd0.2O1.9/3Y-TZP composite electrolytes—Effects of 3Y-TZP particle treatment

Solid oxide fuel cell (SOFC) electrolytes are currently manufactured from yttria stabilised zirconia. A limitation of this material is the high operating temperatures needed for efficient operation, approximately 1000°C, as this can lead to sealing problems and require expensive nickel/chromium alloy auxiliary components. Alternative electrolytes, which allow operation at reduced temperatures, such as gadolinium doped cerium oxide have been suggested. However these ceramic materials often exhibit inferior mechanical properties. In this paper the manufacture and characterisation of composite electrolytes consisting of 3% yttria stabilised zirconia particles in a 20% gadolinia doped ceria matrix is described. Results suggest improvements in strength can be achieved by the composite approach.     

Catalyst nanocomposites templates of carbon nanoribbons, nanospheres and nanotubes

Several attractive types of amorphous or crystalline carbon nanostructure were obtained by a single catalytic process, during natural gas decomposition using different nanostructured catalysts as template. The nanostructured catalyst templates were based on transition metal nanoparticles embedded in carbon matrixes and nanoceramic supports which consist of magnesium oxide doped with ceria rare earths, prepared by the high-energy mechanical milling. The yield and the nature of the nanostructured carbon are strongly influenced by the preparation method of the template and the chemical composition of the catalysts.     

Heavily impregnated ceria nanoparticles with europium oxide: spectroscopic evidences for homogenous solid solutions and intrinsic structure of Eu3+-oxygen environments

We report on the homogeneity, structural and luminescence properties of ceria nanoparticles doped with Eu³⁺. Eu³⁺ in concentration of 1, 5 and 10 wt% was incorporated via wetness impregnation into preformed ceria nanoparticles followed by calcination in air at 1000 °C. A remarkable homogeneity of Eu³⁺-ceria solid solutions is measured for ceria grown by citrate and micro-emulsion methods using Raman, Diffuse Reflectance in UV–Vis, photoluminescence spectroscopies and X-ray diffraction, even for the Eu³⁺ concentration of 10 wt%. The emission properties of all Eu³⁺-doped ceria samples are well-characterized by a two main centre model assigned to perturbed and isolated Eu³⁺ centres. These centres correspond to Eu³⁺ located in the nearest (local symmetry lower than cubic and Eu³⁺-oxygen coordination lower than eight) and next-nearest-neighbour positions (cubic local symmetry and eightfold Eu³⁺-oxygen coordination) to oxygen vacancy, respectively. With increase of Eu³⁺ concentration, both the oxygen vacancy concentration and the relative contribution of the perturbed Eu³⁺ centre to the total emission increase. It is established that the characteristic emission and excitation spectra of the two main Eu³⁺ centres as well as the overall multisite distribution of Eu³⁺ within ceria lattice are intrinsic properties of Eu³⁺-doped ceria since these do not depend on synthesis route, nanoparticle size and Eu³⁺ concentration.